A. Field of the Invention
The invention relates to methods of developing, gumming, gum etching, dampening during printing, cleaning during and after printing, and treatment before storage, of lithographic printing plates, and to compositions therefor which utilize polyacrylamide polymers and blends thereof in place of or as an extender for the traditional gum arabic. The invention also relates to the treated lithographic plates.
B. Description of the Prior Art
Lithographic printing, which is a type of planographic printing, is a well known and established art. In general, the process involves printing from a flat plate or cylinder having substantially no surface relief (hence, the term "planographic"), and depends upon different properties of the image and non-image areas of the surface for printability. In lithography, the image to be reproduced is imparted to the plate by any one of several methods well known to those skilled in the art in such a way that the non-image areas are rendered hydrophilic while the image areas are hydrophobic. A widely practiced technique employs a photosensitive coating for this purpose. Following exposure of the photosensitive coating to imagewise modulated light, the latent image is developed and a portion of the coating is removed from the plate. Next, the plate is treated with a desensitizing solution to render the plate hydrophilic in the areas from which the photo sensitive coating has been removed. During the actual printing process, an aqueous fountain solution is applied to the plate surface. The fountain solution keeps moist all portions of the surface not covered by the hydrophobic image. Furthermore, the fountain solution prevents the plate from scumming, i.e. it prevents the non-image areas from becoming at least partially ink-receptive. The fountain solution may be formulated to gradually etch the surface of the plate just enough to keep the lines sharp and prevent rapid wear. In a conventional system, the fountain solution is applied to the plate by one or more rollers. At least one ink roller coated with an oil-based printing ink then contacts the entire surface of the plate but deposits the lithographic ink only on the image area since the hydrophilic non-image areas repel the ink. Hence, for each impression made during a run, the lithographic plate is first dampened with the aqueous fountain solution and then inked with a lithographic ink. Alternatively, the fountain solution and at least a portion of the oil-based ink are applied to the plate simultaneously with a first roller. In this latter system, other rollers, usually smaller in diameter than the first, may contact the plate subsequently to distrubute the ink more evenly. Finally, the ink image is transferred directly to a paper sheet or other receptive surface to be printed, or to an offset blanket of rubber or synthetic material which in turn transfers the print to the final copy surface.
Gum arabic has long been used in acidic aqueous solutions in the preparation of lithographic plates. Gum arabic has been used, for example, in solutions for developing a latent image; as a desensitizing ingredient in a gumming solution--sometimes in combination with an etchant, in which case the solution is referred to as a gum etch--to make the non-image areas sharply defined and ink repellent, i.e. hydrophilic rather than hydrophobic; in a fountain solution, again sometimes in combination with an etchant, to keep the non-image areas hydrophilic during the press run; as a protective coating during idle periods on the press or even during storage for weeks and months; and in plate cleaner formulation.
Gum arabic is a natural product obtained as an exudate from acacia trees. While the precise structure of gum arabic is not known, the main constituent apparently lies somewhere between hemi-cellulose and the simple sugars. Essentially, it is a mixture of calcium, magnesium, and potassium salts of arabic acid. The average molecular weight of the polymerized arabic acid is in the range of 200,000 to 270,000.
The disadvantages of gum arabic are well recognized in the trade and a suitable substitute has long been sought. As a natural product, it is subject to considerable variation in quality, and it is also prone to contain foreign matter of various kinds so that it must first be purified. The fountain solutions of gum arabic employed during printing to maintain the non-printing areas hydrophilic tend to emulsify the ink. Excessive emulsification weakens the resolution of the printing, causes scumming of the plate, and stripping of the ink from the ink rollers. In addition to the technological shortcomings of gum arabic, the continued assured availability of an adequate supply of the product in the United States is in doubt, since nearly all of the domestic supply is imported from the politically unstable Middle East. U.S. Department of Commerce data projects that for 1975, the quantity of gum arabic imported will be only about one half that imported in 1972, and the unit price has more than doubled in the same period. Thus, a substitute for gum arabic is urgently needed.
Some of the compositions heretofore proposed as gum arabic substitutes for the treatment of image-bearing plates include oxalic acid (U.S. Pat. No. 3,489,561), homopolymers and copolymers of itaconic acid (U.S. Pat. No. 3,507,647), sodium carboxymethycellulose (U.S. Pat. No. 3,166,421), the copolymer of methyl vinyl ether and maleic anhydride (ibid.), polyacrylic acid (U.S. Pat. No. 3,211,686) and sodium and ammonium salts thereof (Japanese Patent Publication No. 49-6561 (1974)), alginic acid and the sodium salt thereof, and polyvinyl alcohol (U.S. Pat. No. 3,607,255). None of these proposed substitutes seems to have realized overwhelming commercial acceptance, at least to this date, as gum arabic continues to be the composition of choice in industry, notwithstanding all of its disadvantages.
Recently, alkaline solutions have been proposed and used with some success as fountain solutions. However, most contain a considerable proportion of surfactants, have a tendency to foam, and cause some emulsification of the ink. Consequently, they are not always suitable.
British Pat. No. 1,055,711 teaches an improved silver halide diffusion transfer process using a light sensitive material comprising a silver halide emulsion layer covered with a water-soluble colloid top layer. The light sensitive material is exposed, developed, and only then brought in contact with the metal comprising the lithographic plate, for transfer of the image to the plate. Included in the list of colloids taught to be suitable for use in the top layer are polyacrylamide and polyacrylic acid. It is also taught that the top layer may contain more than one of the colloids named in the list. However, the patent contains no suggestion to use polyacrylamide or polyacrylic acid--either separately to together--to render hydrophilic the non-image area of the plate once the image has been transferred to the plate: for that purpose, the patentees use carboxymethylcellulose. Indeed, the patent contemplates the disappearance of the colloid of the top layer after the transfer is carried out. Moreover, the patent teaches no preference for mixtures of polyacrylamide and polyacrylic acid over the respective compounds employed separately, thereby suggesting that in the process there described, no particular advantage is obtained by employing the compounds in admixture. In contrast, in the herein described processes for treating lithographic plates already having an image, it has been found to be critical to employ an active polymer component having the proper ratio of both amide and carboxyl groups as illustrated by the numerous Examples and Comparative Runs hereinafter set forth. Also, the weight average molecular weight of the active polymer component is critical in the present process.
In addition, the art contains several suggestions to employ polyacrylic acid-containing or polyacrylamide-containing compositions in various ways as more or less permanent components of the lithographic plate itself, i.e. in compositions which form part of the plate even before the plate contains an image. For example, Leonard et al., U.S. Pat. No. 3,265,504, apply a permanent polyacrylamide-containing coating on the entire surface of the plate by contacting the plate with an abrasive polyacrylamide-containing slurry while simultaneously mechanically brush graining the surface. Dowdall et al., U.S. Pat. No. 3,136,636, employ polyacrylic acid or water soluble homologs thereof in a stratum between the surface of the plate and the diazo coating. Similar teachings are found in several other U.S. patents such as U.S. Pat. No. 3,298,852. In U.S. Pat. No. 3,374,094, Wainer et al. describe a binder for the photosensitive composition for a lithographic plate, the binder containing any of a number of hydrophilic colloids, including polyacrylamide. No attempt has been made, however, so far as the applicants are aware, to utilize polyacrylamide compositions for the treatment of lithographic plates already having an image (latent or developed) thereon, for example, in desensitizing or fountain solutions.